1. Field of the Invention
The present invention relates to a glass panel for a cathode ray tube, in particular, a glass panel for a color cathode ray tube used mainly for a TV or a display device for industrial use.
2. Discussion of the Background
As shown in FIG. 3, a cathode ray tube is generally constituted by a glass bulb which comprises a panel 1 having a picture surface 9 of substantially rectangular shape, a funnel-shaped funnel 2 mounting thereon a deflection coil and a neck 3 for housing an electron gun. The panel 1 comprises a face portion 4 having a picture surface 9 and a skirt portion 10 which is formed contiguous to the face portion at a substantially right angle to thereby form a side wall. As shown in FIG. 4, the skirt portion 10 inclines forward and backward from an outer circumferential portion 5 having the largest diameter in the side wall (hereinbelow, referred to as the largest diameter portion 5), namely, the skirt portion 10 is provided with a forward inclination portion 6 which extends from the largest diameter portion 5 toward the face portion and a backward inclination portion 7 which extends in the opposite direction of the forward inclination portion 6 with respect to the largest diameter portion 5.
The inside of the cathode ray tube is kept in a high vacuum state so that electron beams reach the fluorescent layer. Accordingly, in the glass panel, there is a high deforming energy due to a difference between an inner pressure and an outer pressure. Further, since the cathode ray tube has an asymmetric structure unlike a shell structure having a spherical shape, there are many cases of causing a large scale destruction which invites overall collapse when once a destruction takes place. In particular, when the cathode ray tube receives a mechanical shock, an instantaneous destruction called an implosion phenomenon occurs whereby a large amount of sharp glass fragments may scatter.
To prevent such implosion phenomenon, an anti-implosion band 8 made of steel is generally provided at or in the vicinity of the largest diameter portion 5 formed between the forward inclination portion 6 and the backward inclination portion 7 of the skirt portion 10. The inner circumference of the anti-implosion band 8 is so designed as to be smaller than the outer circumference of the panel 2, and an expansion of the band tightens the face portion 4 from its side. A force given by the tightening produces a compressive force in the face portion 4 whereby occurrence of cracks in the panel and the expansion of the cracks are controlled; the destruction becomes mild, and the scattering of glass fragments resulted from the destruction is prevented.
Further, in the glass panel for a cathode ray tube, the face portion is generally curved in order to suppress the bending of the face portion due to an impact force and to improve the rigidity of the face portion. A curved shape formed in the face portion can convert a mechanical shock to the face portion into a compressive force in the face portion. The effect of the arched shape becomes large as the averaged radius of curvature R of the face portion 4 is smaller, which will be described after.
As another method of increasing the rigidity of the face portion, there is a method of increasing the thickness of the glass panel. Namely, the method is to increase the thickness of the face portion at its central portion (a face central portion) or to increase the thickness of a peripheral portion of the face portion (a face peripheral portion). By thickening the face central portion, the rigidity of the overall face portion can be increased. When the cathode ray tube is brought to a vacuum state, a high stress is occurred in the face peripheral portion. An increased thickness of such portion having a high stress can increase the rigidity of the face portion as well.
On the other hand, there have been employed various measures to improve the quality of picture images displayed in the cathode ray tube as a image displaying device. A curved face portion results a curved picture image displayed thereon. Accordingly, the face portion is desirably flat as possible. Further, since glass absorbs partly light, it is desirable that a difference of thickness between the face central portion and the face peripheral portion is reduced so that a picture image displayed has uniform brightness.
The conventional glass panel for a cathode ray tube was so designed that the face portion was curved, or the face central portion or the face peripheral portion was thickened in order to increase the rigidity of the glass panel and to assure saftiness as described above. In recent years, a demand for applying a function to improving the visibility of the cathode ray tube to the glass panel have been increasing along with an improvement of the performance. For this, demands for making the face portion 4 flat and for reducing the difference of thickness between the face central portion and the face peripheral portion have been increasing.
However, control for increasing the rigidity of the glass panel and assuring saftiness is contrary to control for making the face portion 4 flat and for reducing the thickness. In order to minimize the deformation of a picture image, the face portion should desirably be substantially flat. However, the flat face portion reduces the rigidity, hence, it is impossible to completely prevent the implosion.
Further, when the thickness of the face portion is increased to prevent a danger of the implosion, the weight of the cathode ray tube is increased, which is a big problem in the cathode ray tube. When the thickness of the face peripheral portion is increased, a picture image in the face peripheral portion becomes dark in comparison with the face central portion.